1. Field of the Invention
This invention relates to a pigment dispersing agent used when a pigment is mixed with a coating composition in the manufacture of paints.
2. Related Art Statement
In the manufacture of paints, it is an important factor to mix a pigment with a coating composition. In this case, the pigment to be usually used in the paint is dispersed into only a part of a vehicle constituting the paint together with a proper solvent, diluent or other additives by means of a mill used in the paint industry to form a dispersion paste. Then, the resulting paste is mixed with the remaining amount of the vehicle and another necessary additives to obtain a pigment dispersing composition as a paint. However, a greatest problem of such a pigment dispersing composition lies in that pigment particles are apt to be agglomerated. Such an agglomeration may occur in dispersing step, dissolving step, storing step or painting step. As a result, it is known to cause unfavorable phenomena such as decrease of paint stability, occurrence of troubles in the painting, reductions of color strength, gloss and distinctness of image in the finally obtained paint film, flooding, floating, color change with time and the like. Such a phenomenon that the pigment particles are apt to be agglomerated is explained by London-van der Waals force attracting the particles each other. In order to overcome such an attracting force, it is necessary to provide an adsorption layer onto the surface of the particle, and consequently many pigment dispersing agents and grinding aids are proposed and various methods of improving pigment dispersion are attempted. For instance, there are known (1) a method of dispersing the pigment with a non-ionic, anionic or cationic surfactant, or a wetting agent such as aliphatic polyvalent carboxylic acid or the like as an assistant; (2) a method of dispersing the pigment with an ampholytic substance obtained by reacting a substance having an affinity with a pigment with a substance having a solvent affinity as disclosed in British Patent No. 1,108,261, No. 1,159,252 and No. 1,346,298; (3) a method of decreasing surface tension with a surfactant such as alkyl silicone to prevent the floating; (4) a method of mixing the pigment with a certain substituted derivative of the pigment to conduct pigment dispersion as disclosed in Japanese Patent laid open No. 51-18736; (5) a method of using as a dispersing agent a compound obtained by reacting an organic compound having two or more isocyanate groups, an organic compound having one or more functional group to be reacted with isocyanate group (amino group, hydroxyl group) and a polymer having substantially one functional group to be reacted with isocyanate group as disclosed in U.S. Pat. No. 3,684,711 and British Patent No. 1,393,401; (6) a method of dispersing the pigment with a high molecular weight dispersing agent obtained by reacting a basic substance with a polyester compound as disclosed in EP-A-0208041, Japanese Patent laid open No. 60-166318 and No. 61-174939, and U.S. Pat. No. 4,933,417; (7) a method of dispersing the pigment with a high molecular weight dispersing agent obtained by reacting a basic substance with an acrylic polymer as disclosed in Japanese Patent laid open No. 46-7294; (8) a method of dispersing the pigment with a high molecular weight dispersing agent obtained by reacting a basic substance with an acrylic polymer and a polyester compound as disclosed in EP-A-0358358; and the like.
In the grinding aids or dispersing agents used in the above methods (1)-(3), however, the adsorption layer adsorbed on the particle surface is thin and does not develop a satisfactory stabilizing effect and hence the pigment dispersing performances are not improved. The pigment derivative used in the method (4) is naturally colored, so that it can not generally be used for various pigments. The fundamental thought of the pigment dispersing agent used in the methods (5)-(8) is a technical idea of making a block structure comprised of a polymer portion capable of solvating with a solvent and an anchor portion adsorbed on the pigment as described by A. Topham in Progress in Organic Coatings, vol. 5, (1977) pp 237-243. In case of utilizing such a technique, it is important that the polymer portion capable of solvating with the solvent is excellent in the compatibility with a resin used as a film forming component of the paint film and that the number of adsorption points to the pigment is increased and the adsorption state of the dispersing agent to the pigment is rendered into a tail-like state, whereby an effective steric repulsion layer is formed in the polymer portion of the dispersing agent adsorbed on the pigment to stabilize the dispersed particles, and the like. If the polymer portion solvating with the solvent is poor in the compatibility with the resin added as the film forming component of the paint film, the solvated polymer portion agglomerates to cause the degradation of pigment dispersing performances. Furthermore, when the number of adsorption points to the pigment is small, the resorption of the dispersing agent from the pigment is easily caused to degrade the pigment dispersing performances, while if the adsorption state of the dispersing agent to the pigment is loop, the formation of the steric repulsion layer in the solvated polymer portion is insufficient and the degradation of pigment dispersing performances is caused. In the method (5), the polyester compound made from dialcohol and dicarboxylic acid starting from monoalcohol is used, which includes many compounds such as polyester compound having substantially one hydroxyl group in its terminal, polyester compound having hydroxyl groups in both terminals, polyester compound containing no hydroxyl group. When the dispersing agent is made by using such a polyester compound, the adsorption state to the pigment is tail or loop and hence the satisfactory pigment dispersing performances are not obtained. In the pigment dispersing agent obtained by the method (6), when the dispersing agent is good in the compatibility with the resin added as a film forming component of the paint film, the considerable improvement of the pigment dispersing performances is observed, but when it is poor in the compatibility with the resin, the protection layer of the dispersing agent adsorbed on the pigment agglomerates to undesirably degrade the pigment dispersing performances. Particularly, since a dispersing agent consists mainly of the polyester compound, it is poor in the compatibility with an acrylic resin or the like being important as a painting resin, so that it is difficult to obtain good pigment dispersing performances in case of acrylic resin series paints. In the dispersing agent obtained by the method (7), since an acrylic resin is used in the polymer portion, it acts as a dispersing agent for usual acrylic resin series paints, but it is difficult to provide good pigment dispersing performances in case of polyester series paints because the compatibility with the polyester resin or the like is poor. In the dispersing agent obtained by the method (8), almost of the aforementioned problems are solved, but the yellowing is undesirably caused at the curing in case of thermosetting resin type paints.